1. Field of the Invention
The present invention relates to a method of cleaning a reaction tube and, more particularly, to a cleaning method in which polysilicon, silicon nitride, and silicon oxide films deposited inside the reaction tube are removed by a cleaning gas containing ClF.sub.3. In addition, the present invention relates to a method of forming a film on the surface of a substrate to be processed.
2. Description of the Related Art
Conventionally, in the steps in manufacturing a semiconductor device, a polysilicon-based film such as a polysilicon film or a silicon epitaxial growth film, a silicon nitride film, or a silicon oxide film is formed on a substrate to be processed such as a semiconductor wafer by low-pressure CVD, atmospheric-pressure CVD, or the like.
For example, in the step of forming a polysilicon-based film, a heat treatment apparatus constituted by arranging a heater around a reaction tube consisting of quartz or the like is generally used. For example, a wafer boat in which a large number of semiconductor wafers are stored is loaded in a reaction tube kept at a predetermined temperature, and reactive gases such as an SiH.sub.4 gas, an SiH.sub.2 Cl.sub.2 gas, an Si.sub.2 H.sub.6 gas, and an H.sub.2 gas are supplied into the reaction tube, thereby performing a film forming process of a silicon (Si)-based film. The loading/unloading operations of semiconductor wafers are generally performed to a reaction tube kept at about a process temperature.
When the above film forming process is performed, a polysilicon-based film is deposited on the reaction tube of the heat treatment apparatus or other quartz jigs. When the thickness of the polysilicon-based film deposited on the reaction tube and the like is increased, it is peeled and scattered. The scattered film pieces are adhered on semiconductor wafers. This is a factor of decreasing the yield of the semiconductor wafers. The above problem is also posed in film forming processes of a silicon nitride film and a silicon oxide film.
The film deposited inside the reaction tube is generally removed to be described below at a predetermined frequency. That is, the temperature in the reaction tube is decreased to about an atmospheric temperature, the reaction tube, the quartz members, and the like are removed, and they are wet-washed with a diluted hydrogen fluoride (HF) solution to remove the deposited film.
In the method of cleaning the inside of the reaction tube according to the wet washing, however, the apparatus must be stopped for a very long time to increase/decrease the temperature in the reaction tube and to remove the reaction tube and the like. Therefore, the operational efficiency of the heat treatment apparatus is disadvantageously degraded.
A system such as a load lock system for always holding a reaction tube and a loading part in a vacuum state in a heat treatment apparatus is proposed. In this case, since the heat treatment apparatus is mostly surrounded by a vacuum chamber, an object subject to cleaned such as a reaction tube consisting of quartz cannot easily be removed from the heat treatment apparatus. In recent years, the diameter of a semiconductor wafer tends to be increased, and the size of a heat treatment apparatus is increased accordingly. In this case, since the sizes of a reaction tube main body, a quartz jig, and the like are increased, they cannot easily be removed from the apparatus to wash them. As described above, in the wet cleaning, the heat treatment apparatus must be disassembled into parts to remove objects to be cleaned. Therefore, the wet cleaning cannot easily cope with an increase in integration density of a semiconductor device and an increase in size of a wafer in recent years.
In order to solve the above problem, dry cleaning in which an etching gas flows into a reaction tube to clean the inside of the reaction tube has been attempted. According to this method, an etching gas such as a carbon tetrafluoride (CF.sub.4) gas, a nitrogen trifluoride (NF.sub.3) gas, or a sulfur hexafluoride (SF.sub.6) gas is supplied to a reaction tube, and a plasma is produced in the reaction tube, so that a film deposited inside the reaction tube is removed by etching. More specifically, it has been reported that cleaning can be performed without producing a plasma by using a chlorine trifluoride (ClF.sub.3) gas as an etching gas.
However, as the ClF.sub.3 gas has high reactivity, a material such as quartz constituting a reaction tube and a jig incorporated in the reaction tube are etched by the ClF.sub.3 gas under a high-temperature condition. For this reason, ClF.sub.3 haven't been able to be considered to be used at a temperature of 400.degree. C. or more, and cleaning is generally performed at a temperature of 400.degree. C. or less. Therefore, in the film forming step under a temperature condition of 400.degree. C. or more, and more particularly, 600.degree. C. or more, the temperature in the reaction tube must be decreased to 400.degree. C. or less by spontaneously radiation to perform cleaning using the ClF.sub.3 gas. In addition, the temperature in the reaction tube must be reset to restart the film forming step. As a result, the cleaning requires a long time, and the operational efficiency of the heat treatment apparatus is degraded.
As described above, both the conventional wet-cleaning and dry-cleaning methods require cumbersome operations, and a long time is required to perform these methods. For this reason, the methods are not suitable for frequently performing cleaning in the film forming step to always keep the inside of the reaction tube clean. In accordance with an increase in integration density of a semiconductor device in recent years, an operation environment is required to be more clean. However, this requirement cannot easily be achieved by the conventional cleaning methods.